In 98 anime his motivation for that is largely centered around grief. If he kills a person, then the last thing of Rem he has would be lost. And the way Rem didn't even talk to him directly about her personal convictions back in his childhood, Vash saw her pacify a violent crew member (presented as a rather unpleasant guy in fact) and internalised it as the ultimate essence of her.

In Trimax the flashback goes quite differently. There's no violent crew members present - the source of violence in that scene is Vash himself, and I think this shift of perspective is vital to understand the entire framework of the manga. Rem talks directly to him, and the final decision on whether he would actually commit murder is ultimately left to him. And then he doesn't. And him and Rem actually talk, have a heart to heart, Rem explains to him her own trauma and motivation for joining the SEEDS crew. It is very raw, intimate, and Vash comes out of it with an understanding of why killing a person is wrong - because all that interiority, all that potential to live again, it will be lost forever if a death occurs. And having been the one who lashed out, attacked another person (and himself!) out of his own anger, fear and grief, he also gains a perspective on how violence is perpetuated between people. His pacifism in Trimax is no longer about Rem, but about his own experience with facing violence and perpetuating it in turn.

Man, I love Trimax.